Effects of pH on Simultaneous Nitrogen and Manganese Removal in MBBR
Impact of pH on MBBR Performance
pH plays a critical role in the efficiency of Moving Bed Biofilm Reactors (MBBR) by directly influencing microbial activity and biochemical reaction rates. As a key environmental factor, pH variations affect:
- Biofilm community structure - Shifts in pH alter the dominance of nitrifying/denitrifying bacteria and manganese-oxidizing microorganisms.
- Enzyme activity - Optimal pH ranges govern the performance of nitrite oxidoreductase (pH 7-8) and manganese oxidase (pH 6-7).
- Redox reaction kinetics - pH determines the equilibrium between Mn²⁺/Mn⁴⁺ transformations and nitrogen conversion pathways.
- Precipitation potential - Higher pH (>8) promotes Mn²⁺ oxidation and phosphate precipitation, while acidic conditions (pH<6) may inhibit these processes.
The system demonstrates remarkable adaptability, with certain microbial populations maintaining functionality across wide pH ranges (5-9), though optimal removal efficiencies for different contaminants occur at specific pH levels.
MBBR Performance Under Different pH Conditions
A recent study conducted by a Chinese university investigated the performance of Moving Bed Biofilm Reactor (MBBR) systems under varying pH conditions (pH 5-9), and also Under the condition of an influent Mn²⁺ concentration of 10 mg·L⁻¹. The influent and effluent concentrations of NH₄⁺-N, TN, TP, COD, Mn²⁺, NO₂⁻-N, and NO₃⁻-N during operational Phases I-V are summarized below.
(1)NH₄⁺-N Removal Efficiency
The MBBR demonstrated consistently high NH₄⁺-N removal across all pH levels, with average efficiencies of 96.22% (pH 5), 98.89% (pH 6), 98.70% (pH 7), 98.65% (pH 8), and 96.69% (pH 9). These results indicate robust nitrification performance (>96% efficiency) regardless of pH variation. While the removal efficiency initially increased from pH 5 to 6 (peaking at 98.89%) before gradually decreasing at higher pH levels, the overall impact of pH on NH₄⁺-N removal was minimal. This suggests strong adaptability of nitrifying bacteria within the biofilm to pH fluctuations.
(2)TN Removal Efficiency
Total nitrogen removal exhibited significant pH dependence:
- pH 5: 40.13%
- pH 6: 42.66%
- pH 7: 49.20%
- pH 8: 52.74%
- pH 9: 69.79% (peak performance)
The 29.66% improvement from pH 5 to 9 highlights enhanced denitrifying microbial activity under alkaline conditions.
(3)COD Removal Efficiency
COD removal followed a bell-shaped curve:
- Optimal neutral pH: 94.27% at pH 7
- Decline at extremes:
- pH 5: 90.85%
- pH 9: 53.81%
The sharp drop at pH>7 suggests inhibition of heterotrophic bacteria in alkaline environments.
(4)Mn²⁺ Removal Efficiency
Mn²⁺ removal was most efficient at near-neutral pH:
- pH 6: 95.74% (optimal for Mn²⁺→MnOx oxidation)
- pH 5/9: <60% efficiency
This correlates with manganese-oxidizing microbial activity trends.
(5)TP Removal Efficiency
Phosphorus removal improved linearly with pH:
- pH 5: 20.70% → pH 9: 51.76%
The lowest effluent TP (2.80 mg/L at pH 9) indicates alkaline-favored PAOs activity.
(6)NO₃⁻-N & NO₂⁻-N Dynamics
- NO₃⁻-N minimization at pH 9: 5.89 mg/L (vs. 11.63 mg/L at pH 5)
- Stable NO₂⁻-N accumulation (0.16–0.19 mg/L) across all phases
This confirms synergistic nitrification-denitrification at alkaline pH.
Conclusion
Under the condition of an influent Mn²⁺ concentration of 10 mg·L⁻¹, this study further investigated the impact of varying pH levels on MBBR performance for wastewater treatment. The results demonstrated that when the influent pH was increased to 9, the average removal efficiencies of NH₄⁺-N, TN, and TP reached 96.69%, 69.79%, and 51.76%, respectively. Compared to Phase I (pH 5), the removal efficiencies of TN and TP increased significantly by 29.66% and 31.06%, respectively.
Key Findings
1. Optimal Performance at pH 9
- Highest N&P Removal: The MBBR exhibited its best denitrification and phosphorus removal capabilities under alkaline conditions (pH 9), with minimal NO₃⁻-N generation and near-complete NH₄⁺-N conversion.
- Enhanced Microbial Activity: The Effective Total Surface Area (ETSA) of the biofilm increased proportionally with pH (7-9), peaking at pH 9, indicating superior metabolic activity under alkaline conditions. This is likely due to the abundance of free hydroxide ions (OH⁻), which enhance simultaneous nitrification-denitrification (SND) efficiency.
2. Mn²⁺ Removal Mechanism
- Extracellular Adsorption Dominance: Across all phases (I-V), over 75% of Mn²⁺ removal was achieved via extracellular adsorption by biofilm microorganisms.
3. Microbial Community Dynamics
- Alkaline-Favored Denitrifiers: Key denitrifying genera such as Comamonas and Hyphomicrobium showed increased relative abundance at higher pH levels, confirming their adaptation to alkaline environments.
- Comamonas aquatica LNL3 demonstrated exceptional metabolic versatility, converting both NH₄⁺-N → NO₂⁻-N and NH₄⁺-N → N₂.
- Enhanced Biodiversity at pH 9: Unique Operational Taxonomic Units (OTUs) increased from 2 (pH 5) to 13 (pH 9), reflecting greater microbial richness under alkaline conditions.
4. Functional Implications
- Synergistic Nutrient Removal: Alkaline pH (9) promoted the activity of polyphosphate-accumulating organisms (PAOs) and denitrifying bacteria (e.g., Acinetobacter), optimizing simultaneous N-P removal.
- Process Stability: The MBBR maintained robust Mn²⁺ adsorption (>75%) regardless of pH shifts, highlighting system resilience.
Practical Implications
- Recommended Operational pH: 8.5–9.0 for maximal TN/TP removal in Mn²⁺-amended MBBR systems.
- Microbial Management: Bioaugmentation with Comamonas or Hyphomicrobium strains could further enhance denitrification in alkaline reactors.